Fuse elements that fuse under their own heat build-up when current greater than a current rating flows and cut off the current path have been used in the related art. For example, holder-type fuses in which solder is enclosed within a glass tube, chip fuses in which an Ag electrode is printed on the surface of a ceramic substrate, screw-in or plug-in type fuses in which a partially-thinned copper electrode is incorporated into a plastic case, and the like are often used as fuse elements.
However, problems have been identified with the above known fuse elements, namely that the elements cannot be reflow surface mounted, the rated currents are low, and the speed at which the fuse blows drops in a case where the size is increased to increase the current rating.
Meanwhile, in the case of a fast-acting fuse device for reflow mounting, a high-melting point solder that contains Pb and has a melting point of 300° C. or higher is generally preferable with respect to the fusing characteristics, such that the solder is not melted by the reflow heat. However, in RoHS directives and the like, Pb-containing solder is permitted only in limited situations, and the demand for Pb-free solders is expected to intensify in the future.
Based on such demand, a fuse element 100 in which a high melting point metal layer 102 such as silver or copper is layered on a low melting point metal layer 101 such as Pb-free solder is used, as illustrated in FIG. 45. According to this fuse element 100, reflow surface mounting is possible, which provides superior mountability on a fuse device; a high melting point metal covering is used, which raises the current rating and makes it possible to handle high currents; and furthermore, an erosion effect arises in the high melting point metal at the time of fusing due to the low melting point metal, which makes it possible to break the current path.